One type of a GMR read head is a sandwich structure comprising two uncoupled ferromagnetic layers separated by a nonmagnetic metallic electrically conducting spacer layer, typically copper (Cu), in which the magnetization direction (magnetic moment) of one of the ferromagnetic layers is fixed or pinned, while the magnetization direction of the free or sensing ferromagnetic layer is free to rotate. This type of GMR device is referred to as a spin valve magnetoresistive sensor in which only the free or sensing ferromagnetic layer is free to rotate in the presence of an external magnetic field in the range of interest for the sensor. The basic spin valve magnetoresistive sensor is described in IBM's U.S. Pat. No. 5,206,590.
A magnetic tunnel junction (MTJ) device has two ferromagnetic layers separated by a nonmagnetic electrically insulating layer, called the tunnel barrier layer, which is typically formed of alumina. One of the ferromagnetic layers is a pinned layer whose magnetization direction is oriented in the plane of the layer but is fixed or pinned so as not to be able to rotate in the presence of an applied magnetic field. The pinned ferromagnetic layer may be pinned by interface exchange biasing with an adjacent antiferromagnetic layer, while the free ferromagnetic layer has its magnetization direction capable of rotation relative to the pinned layer's magnetization direction. The tunneling current that flows perpendicularly through the insulating tunnel barrier layer depends on the relative magnetization directions of the two ferromagnetic layers. MTJ devices have applications for use as memory cells in magnetic memory arrays and as magnetoresistive read heads in magnetic recording devices.
The spin valve magnetoresistive sensor has been improved by substitution of one or both of the free and pinned ferromagnetic layers with a laminated layer comprising two ferromagnetic films antiferromagnetically coupled to one another in an antiparallel orientation by a antiferromagnetically coupling (AFC) film. This laminated structure is magnetically rigid so that when used as the free ferromagnetic layer the two antiparallel films rotate together. These improved spin valve sensors are described in IBM's U.S. Pat. Nos. 5,408,377 and 5,465,185, which are incorporated herein by reference. The MTJ device has also been improved by substitution of this type of laminated layer for the pinned layer, as described in IBM's U.S. Pat. No. 5,841,692, which is incorporated herein by reference.
This type of laminated structure is based upon the discovery of the oscillatory coupling relationship for selected material combinations of ferromagnetic films and metallic AFC films, as described in detail by S. S. P. Parkin et al., Phys Rev. Lett., Vol. 64, p. 2034 (1990). The sign and strength of the interlayer coupling for thin AFC films is important for these laminated structures. Of all the transition metal materials ruthenium (Ru) was shown to be the most useful for obtaining strong antiferromagnetic coupling in the limit of very thin AFC films. Ru displays very strong antiferromagnetic coupling between cobalt (Co), cobalt-iron (Co--Fe) and nickel-iron (Ni--Fe) ferromagnetic films even when just .about.3 .ANG. thick. This makes Ru very useful to form pairs of antiparallel oriented ferromagnetic films, which are useful for a variety of applications to reduce the net magnetic moment of the ferromagnetic layer, for example, as described in the above-cited patents relating to spin valve magnetoresistive sensors and MTJ devices. It is also useful for many applications that the thickness of the AFC film be thin, either because this takes up less space, or because the AFC film must not significantly increase the conductance of the laminated layer. Thus Ru is particularly useful, because it is both very thin and of relatively high resistivity. By contrast, Cu is not as useful because it must be significantly thicker in order to obtain antiferromagnetic coupling and is one of the most conducting elements.
It has also been shown that the magnitude of the interlayer coupling increases with increasing number of d electrons (i.e. for elements varied across the periodic table from left to right) and is systematically higher for 5 d than for 4 d and for 4 d than for 3 d elements (i.e. for elements varied along a column in the periodic table from bottom to top). Moreover, it has also been shown that the strength of the interlayer coupling decreases with increasing AFC film thickness. See "Systematic Variation of the Strength and Oscillation Period of Indirect Magnetic Exchange Coupling through the 3 d, 4 d, and 5 d Transition Metals", S.S.P. Parkin, Phys Rev. Lett., Vol. 67, pp. 3598-3601 (1991). Thus Ru is especially useful because it gives rise to very strong antiferromagnetic coupling both because it exhibits antiferromagnetic coupling for very thin layers and also because of its position in the Periodic Table of elements. Finally, Ru is especially useful because it exhibits strong antiferromagnetic coupling and oscillatory interlayer coupling for a wide range of ferromagnetic materials. By contrast, rhodium (Rh), which would appear to exhibit nominally higher interlayer coupling strengths when its oscillatory behavior is extrapolated to very thin Rh layers, is actually ferromagnetic at .about.3 .ANG. and generally requires AFC films .about.7-8 .ANG. thick for antiferromagnetic coupling. In addition, Rh is more conducting than Ru and the magnetic properties of the laminated layer containing the Rh AFC film are much more sensitive to the material of which the ferromagnetic films are formed.
While Ru has many attractive properties, including large antiferromagnetic coupling for very thin Ru layers, for certain applications the antiferromagnetic coupling strength is too large. For spin valve magnetoresistive heads it is very useful to form the exchange biased pinned ferromagnetic layer as this type of laminated layer with a Ru AFC film, as described in the '185 patent. This reduces the net magnetic moment of the pinned ferromagnetic layer (in magnetic fields below the field required to saturate the magnetic moments of the two ferromagnetic films forming the laminated pinned layer) so that the magnetostatic field which the free ferromagnetic layer is subjected to is reduced. In addition, the reduced magnetic moment of the pinned ferromagnetic layer makes the exchange coupling field exhibited by the antiferromagnet on the pinned layer considerably larger. Thus the use of this laminated pinned layer makes this layer magnetically more stable against magnetic fields, especially at higher temperatures. However, the antiferromagnetic coupling strength provided by thin Ru AFC films is so large that it is difficult to saturate the magnetic moment of the laminated pinned layer without applying very large magnetic fields of the order of 10-20 kOe. In a manufacturing environment when devices are formed from films deposited on large area substrates (typically circular or square substrates 5-8 inches in diameter or width) it is very difficult and expensive to build a magnet powerful enough to give a uniform magnetic field .about.10-20 kOe in strength across such large areas. It is also necessary to reset the direction of the exchange bias field in a manufacturing environment during processing of the heads. Moreover, it is also sometimes useful to be able to reset the direction of the exchange pinning of the pinned ferromagnetic layer in an operating spin valve (or MTJ) head, as described for example in IBM's U.S. Pat. No. 5,650,887. For Ru antiferromagnetic coupling layers, very large magnetic fields must be applied to reset the exchange bias field either for large wafers during processing or in spin valve heads during their operation.
What is needed is a magnetic device where one or more of the ferromagnetic layers is a laminated antiferromagnetically coupled layer having an AFC film formed of material which is no more conducting than Ru yet which gives lower and variable antiferromagnetic coupling strengths.